Machine Design

Molding a better plastic nut

Injection molding can bolster performance and cut the cost of power-transmission nuts.

Kyle Bober Haydon Switch and Instrument Corp.
Waterbury, Conn.

The reinforcing fibers inside a injectionmolded plastic nut don't penetrate the thread surface. Its machined counterpart, on the other hand, will often have cut and exposed reinforcing fibers at thread surfaces, which will cause friction and accelerate wear as the nut touches the leadscrew.

The graph compares the break-in-wear curves of an injection-molded thread versus its machined counterpart.

The performance of power-transmission nuts made from plastics can vary drastically depending on how they are manufactured. The most common processes are machining from bar or sheet stock and injection molding from compounded resins. Injection molding is the most versatile. It inherently allows more design freedom and can accurately produce profiles not possible or too expensive to machine. Additionally, injection molding produces parts with highquality surface finishes and good dimensional control without costly secondary finishing operations. And, it lets designers spec tighter leadscrew clearances (tolerances) than economically possible with machining.

High-performance polymers and special additives and fillers also let designers bolster structural integrity or add internal lubricity. Carbon fibers, fiberglass, and Kevlar fillers boost nut strength and thus load-carrying capacity. Silicon and PTFE additives,on the other hand, form lubricated layers at the thread, lessening friction at the nutscrew interface so the nut lasts longer.

Machining operations often degrade surface finishes. Tapping, for example, can produce burrs and rough-cut edges that hamper performance and cause friction at screw-nut interfaces. It takes longer to produce even thread contact with rough-cut edges. There is a longer break-in wear period for the nut.

Molded threads also make quicker and more even contact than machined threads. This reduces the initial backlash ( clearance between the screw and nut thread) in the nut. Clearances between the nut and leadscrew are inherent in the thread design. These clearances affect how the nut fits the leadscrew.

Tapping also yields inconsistent clearances (tolerances) in the thread. Special oversized taps, for example, are necessary to cut threads in acetal copolymers and other plastics that may shrink afterwards or flow away from the tap rather than actually be cut. Additionally, tapping produces residual stresses that can make threads creep over time and thus make the nut put more drag torque on the leadscrew.

Molding, on the other hand, introduces less inherent stress and, therefore, nut threads will have tighter and more consistent tolerances. Consistent tolerances make the nut thread more accurate and boost its performance.

It's almost a given that the bestperforming products tend to come at a premium. However, molding can reduce part costs. There are three factors to evaluate when determining cost: materials, cycle times, and labor setup. Molding brings low cycle time per part. It also takes less time to set up an injection-molding machine than to set up a machining center that does the same thing. In addition, depending on the nut shape, machining can produce a significant amount of waste chips. In contrast, molded nuts have minimal material waste. Any waste generated can often be reground and used again.

On the downside, however, the initial tooling costs for injection molding do not justify making low-volume runs or prototype parts. It is more practical to use molding for high-volume runs or those needing performance properties or profiles not economically possible with machining.

Haydon Switch and Instrument Corp., (203) 756-7441,

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